Group By no longer works

Today, the Group By function in one universe has stopped working.

No changes have been made to the universe or the environment, and it has only appeared in one universe. This means that any reports which run from the universe with the issue, fail with the error 'Database Error: ORA-00937: not a single-group group function. (IES 10901).'

After spending a large amount of time searching forums and the internet as well as trying different methods to resolve the issue, none have been succesful.

Any help or guidance here would be much appreciated!

Thanks in advance